Today, various surface acoustic wave devices utilizing surface acoustic waves which propagate along a surface of an elastic material are under development.
There are known, as materials for a piezoelectric substrate for surface acoustic wave devices, piezoelectric single crystals such as lithium niobate (LiNbO.sub.3), piezoelectric ceramic such as lead zirconate titanate (PZT) or a combination of a non-piezoelectric substrate and a piezoelectric layer such as zinc oxide (ZnO). Among those, lithium niobate has the advantages that its electromechanical coupling factor K is large and that propagation loss of a surface wave is small but has a disadvantage that its temperature coefficient is large. Piezoelectric ceramic has an advantage that its electrocmechanical coupling factor K is large but has a disadvantage that the propagation loss of a surface wave increases as its frequency becomes high, because it is a sintered material. Further, since said piezoelectric single crystal and piezoelectric ceramic have their own specific functions, respectively, they may be used only in rather limited ways and are not suitable for combination with integrated circuit technology to make devices with desired functions.
On the other hand, when said piezoelectric layer is used, the element is composed, as shown in FIG. 1, of a zinc oxide layer 2, for example, deposited on a non-piezoelectric substrate such as a silicon substrate 1, and electrodes 3 and 4 provided on the zinc oxide layer 2. This permits selected semiconductive elements to be formed on the silicon substrate 1, leading to realization of devices with desired functions.
However, surface-acoustic-wave element including such a piezoelectric layer has a disadvantage that its electromechanical coupling factor is smaller than said piezoelectric single crystal and piezoelectric ceramic, causing inefficiency of its function. Further, it has another disadvantage in that its temperature coefficient is rather large, which make it unsuitable for use in devices in which signal the delay time a is an important factor.